Recently, in line with the reinforcement of environmental regulations at home and abroad, material industry as well as automobile industry is searching for a solution for fuel economy as energy saving and pollution prevention measures.
In general, as a solution for improving fuel economy, improvement of engine efficiency, reduction of running resistance, and weight reduction in automotive bodies have been considered. However, the most effective method is the weight reduction in automotive bodies, and it is known that an improvement in fuel economy of about 10% can be achieved by a weight reduction of 10%.
Accordingly, research to replace conventional steel with an aluminum alloy having low weight and high strength has been actively conducted.
However, a high-strength aluminum sheet currently manufactured has a high cost structure because the sheet is manufactured through complex processes, such as a heat treatment after ingot casting, hot rolling, and cold rolling, and the control of the sizes of crystallization phases and inclusions may be difficult because the ingot is cast in the form of a large slab.
Thus, in order to widely apply an aluminum alloy to automotive components, there is a need to improve various properties, such as specific strength and formability, and simultaneously, develop low-cost manufacturing process technology which may secure cost competitiveness by minimizing an increase in cost accompanying during the replacement of the conventional steel.
As the manufacturing process technology of a metal sheet, twin-roll casting is a process which can directly manufacture a sheet from a melt by the unification of two processes such as casting and hot rolling, wherein the twin-roll casting has many metallurgical advantages because it is possible to control fine cast structure and crystallization phases, which are difficult to be obtained by typical ingot casting due to a high cooling rate during the casting.
With respect to conventional twin-roll casting, it has been introduced to manufacture low-alloyed aluminum alloy sheets, in which microstructural control is relatively easy due to a small deviation in temperature range of a solid-liquid coexistence region, at an economic cost. However, recently, research for manufacturing high-strength high-alloyed aluminum sheets by precise process control has been attempted.
However, with respect to twin-roll casting of high-alloyed aluminum alloy, a deviation in the size of dissolved elements and precipitates between a surface portion and a center portion of a sheet may occur due to the difference in cooling rates, and this causes a microstructural inhomogenization which can deteriorate mechanical properties of the sheet during a post-processing heat treatment.
Therefore, microstructural control by appropriate post-processing and heat treatment process is required.
A composition and tensile properties of an AA7075 alloy sheet, which are currently commercially used, are respectively present in Tables 1 and 2 below.
TABLE 1Alloy composition/wt %AlloyAlZnCuMgCrMnTiSiFeOthers7075-T4bal.5.10-6.101.20-2.002.10-2.900.18-0.28≤0.30≤0.20≤0.40≤0.50≤0.15
TABLE 2Tensile propertiesAlloyYield strength/MPaTensile strength/MPaElongation/%7075-T420539512
As the prior art related to a method of manufacturing an aluminum alloy sheet, Korean Patent Application Laid-Open Publication No. 10-2012-0135546 discloses a method of manufacturing a scandium-added aluminum alloy including performing a solution treatment and natural aging for increasing strength and elongation of the scandium-added aluminum alloy. Specifically, disclosed is the method of manufacturing a scandium-added aluminum alloy including performing a solution treatment for controlling a recrystallized fraction and the amount of vacancy clusters formed, and increasing elongation after casting and performing a homogenization treatment on an aluminum-zinc-(magnesium)-(copper)-(zirconium)-(titanium)-scandium (Al—Zn—(Mg)—(Cu)—(Zr)—(Ti)—Sc) alloy; and performing natural aging for increasing strength by being precipitated as Guinier-Preston (GP) zones during holding at room temperature.
However, in the case that an aluminum alloy is prepared by the above manufacturing method, a degree of improving elongation may be insignificant.
What is needed, therefore is a method of manufacturing an aluminum-zinc-magnesium-copper alloy sheet having improved elongation by controlling a microstructure of the aluminum alloy sheet.